Resetting the plugins folder by FTP and PHPMyAdmin (alternately, not simultaneously), didn't resolve the issue.

I have an identical site on my local testing server, with almost all of the plugins on my remote site active (plugins such as IntenseDebate and Jetpack are not running locally as they require the plugins in the wild), and the feed works as desired. Could it be a problem with the WordPress software and the feed generation script? I have the AddToAny plugin installed and the feed option does not work, but importing the feed into Google Reader seems to return the valid posts. Feedburner (owned by Google) also returns that it is an invalid feed with the same issue.